1. Field of the Invention
The present invention relates to a semiconductor light emitting device having a plastic encapsulant, and more specifically, relates to a plastic encapsulated semiconductor light emitting device having a light emitting element and a light-permeating cover which can convert the wavelength of the light emitted from the light emitting element into a light of different wavelength to irradiate it outside of the cover.
2. Description of the Prior Art
A prior art semiconductor light emitting device as shown in FIG. 2 has a plurality of leads 1, 2 and a semiconductor light emitting element 3 secured with an adhesive agent such as solder (not shown) on a bottom surface 6 of a dished or cup-like portion (header) 5 formed at the end of the lead 1. A lead wire 4 is connected between an electrode formed on a top surface of the semiconductor light emitting element 3 and a metal post of the lead 2 for electrical connection. A plastic encapsulant 9 is formed by the well known transfer molding or casting method to seal each one end of the leads 1, 2, the semiconductor light emitting element 3 and the lead wire 4. The plastic encapsulant 9 comprises a sealing portion 10 formed into a substantially cylindrical shape, and a lens portion 11 formed into a substantially hemispherical shape integral with the sealing portion 10. The plastic encapsulant 9 mainly contains an epoxy resin or the like having a light permeability, and as required, some additives may be mixed in the epoxy resin such as a scattering agent such as silica or the like and a pigment of a certain non-light emitting substance, however, no fluorescent particle is added to the plastic encapsulant 9.
Attached to the plastic encapsulant 9 is a light-permeating cover 12 which comprises a lower portion 13 formed into a substantially cylindrical shape, and an upper portion 14 formed into a substantially hemispherical shape integral with the lower portion 13. The lower and upper portions 13, 14 have respectively their shapes matched to the sealing and lens portions 10, 11 of the plastic encapsulant 9. When the cover 12 is attached to the plastic encapsulant 9, the lens portion 11 is inserted into the opening 15 provided at one end of the lower portion 13 to fit the plastic encapsulant 9 in the cover 12 so that an inner surface of the cover 12 is in the close and tight contact with an outer surface of the plastic encapsulant 9. Therefore, the cover 12 attached to the encapsulant 9 does not easily come off out of the plastic encapsulant 9 although an external force such as vibration is applied to the cover 12.
When the semiconductor light emitting element 3 is electrically operated, it produces a light, a part of which is reflected on side surfaces of the dished portion 5 formed in the upper portion of the lead 1. However, the other part of the light is emitted from the top of the semiconductor light emitting element 3 without being reflected on the side surfaces of the dished portion 5. Both of these two lights are condensed or converged by the lens portion 11 of the plastic encapsulant 9 to irradiate it outside of the plastic encapsulant 9.
When the light emitting element 3 comprises for example a GaAlP, GaP or GaN semiconductor, it generates a light of red, green or blue color respectively. When the light emitting element 3 comprises a GaAs semiconductor, the semiconductor light emitting device emits infrared radiation. If no cover 12 is attached to the semiconductor light emitting device, the light projected from the plastic encapsulant 9 offers an extremely sharp directivity of emission light intensity around the tip of the lens portion 11 of the plastic encapsulant 9 with respect to angular distance from the central axis of the plastic encapsulant 9 as shown in FIG. 3 which exemplifies that the light is practically not radiated over the angular range of xc2x130 degrees. The color of the light emitted from this semiconductor light emitting device is determined by the inherent emission light wavelength of the semiconductor light emitting element 3.
On the other hand, when the cover 12 is attached to the plastic encapsulant 9, the light of the semiconductor light emitting element 3 passes through the plastic encapsulant 9, and then is radiated outside of the cover 12 with scattering of the light by the fluorescent particles 16 dispersed in the cover 12.
The fluorescent particles 16 refer to substances which absorb a light irradiated thereto, while radiate a light of the wavelength different from that of the absorbed light. Therefore, the light emitted from the cover 12 offers an extremely wide directivity of emission light intensity as shown in FIG. 4, and the scattered light is spread out to the entire angle range of xc2x190 degrees. Because the fluorescent particles 16 are excited by the light irradiated from the semiconductor light emitting element 3 through the plastic encapsulant 9, the fluorescent particles 16 radiate lights of the wavelength different from that of the light from the semiconductor light emitting element 3. A part of the light emitted from the semiconductor light emitting element 3 passes through an area of the light-permeating cover 12 without fluorescent particles 16 so that it is then directly irradiated outside of the cover 12 in the absence of excited fluorescent particles. Consequently, the unchanged light emitted from the semiconductor light emitting element 3 is mixed with the wavelength-converted light emitted from the fluorescent particles 16 to emit the light of the mixed color wavelengths outside of the cover 12.
In the prior art semiconductor light emitting device, the cover 12 has approximately uniform thickness in the whole area with neglect of the production errors and the uneven shrinkage of the plastic materials. The light from the semiconductor light emitting element 3 is intense at the tip of the lens portion 11 so that there is a large likelihood of the light component which does not impinge on the fluorescent particles 16 and passes through and outside of the cover 12 without the wavelength-conversion of the major component of emission light from the semiconductor light emitting element 3. On the other hand, only weak light from the semiconductor light emitting element 3 passes through the sealing portion 10 of the plastic encapsulant 9, and the major portion of the weak light impinges on the fluorescent particles 16 for wavelength-conversion. Therefore, when the cover 12 is applied to the plastic encapsulant 9 for the surface light emission indication with scattered light, wavelength of the light irradiated outwardly from the semiconductor light emitting device varies with the light emission angle with respect to a central axis of the plastic encapsulant 9 in the prior art semiconductor light emitting devices. Thus, for example, an undesirable problem arises that the emission light color changes depending upon the area of the light emitting surface so that the visible emission light does not have a specific, uniform and exact color. Obviously, this defect would give rise to bad impression to people that the semiconductor light emitting device is of poor quality.
Therefore, an object of the present invention is to provide a plastic encapsulated semiconductor light emitting device capable of emitting light with nearly uniform light wavelength at any angular distance from the central axis of the plastic encapsulant with a light-permeating cover attached thereon.
The plastic encapsulated semiconductor light emitting device according to the present invention comprises a plurality of leads 1, 2; at least one of which is formed with a dished portion 5; a semiconductor light emitting element 3 attached on the bottom surface of the dished portion 5 for electrical connection of the light emitting element 3 across the leads 1, 2; a plastic encapsulant 9 for sealing each one end of the leads 1, 2 and the semiconductor light emitting element 3; and a light-permeating cover 20 preferably removably attached to an outer surface of the plastic encapsulant 9 with fluorescent particles 16 contained in the cover 20. The plastic encapsulant 9 comprises a sealing portion 10 formed into a substantially cylindrical shape, and a lens portion 11 formed into a substantially hemispherical shape integral with the sealing portion 10. The cover 20 comprises a lower portion 21 formed into a substantially cylindrical shape matched to the sealing portion 10 of the plastic encapsulant 9, and an upper portion 22 formed into a substantially hemispherical shape integral with the lower portion 21 and matched to the lens portion 11 of the plastic encapsulant 9. A blue light irradiated from the semiconductor light emitting element 3 passes through the plastic encapsulant 9 and reaches the fluorescent particles 16 in the cover 20 to activate or excite the fluorescent particles 16 for wavelength-conversion of the light emitted from the semiconductor light emitting element 3. As a result, the fluorescent particles 16 generates a white light from the cover 20 with its wavelength different from that of the light from the semiconductor light emitting element 3. The number of the fluorescent particles 16 is greater in the upper portion 22 of the cover 20 than in the lower portion 21 to produce the substantially homogeneous white light from any outer surface of the cover 20.
As the semiconductor light emitting element 3 emits intense light of the extremely sharp directivity at the tip of the lens portion 11 of the plastic encapsulant 9, a greater number of fluorescent particles 16 are positioned in the cover 20 near the tip of the lens portion 11 of the plastic encapsulant 9 to increase the amount of light impinging on the fluorescent particles 16. Therefore, the amount of light impinging on the fluorescent particles 16 is substantially balanced to that of light permeating through the cover 20 but deviating from the fluorescent particles 16. The cover 20 includes a smaller number of fluorescent particles 16 in the vicinity of the sealing portion 10 of the plastic encapsulant 9 which receives the light of the lower intensity emitted from the semiconductor light emitting element 3. One part of the light emitted from the semiconductor light emitting element 3 deviates from and does not impinge on the fluorescent particles 16 and passes through the cover 20 to irradiate the wavelength-unchanged light. The other part of the light emitted from the semiconductor light emitting element 3 impinges on and excite the fluorescent particles 16 to generate the wavelength-converted light, and then, there would be substantially constant ratio of the amount of the wavelength-converted light to the amount of the unchanged light. Accordingly, the semiconductor light emitting device can emit light with the nearly uniform wavelength at any angular distance from the central axis of the semiconductor light emitting device.
In an embodiment of the present invention, the dished portion 5 has inclined side surfaces 7 for surrounding the bottom surface 6. The thickness of the cover 20 is the greatest at the vertex of the upper portion 22, and is gradually reduced toward the lower portion 21 so that the thickness of the cover 20 in the upper portion 22 is greater than the thickness of the lower portion 21.